The present invention relates to glass tubes and lamp elements for use in high-pressure discharge lamps. In particular, the present invention relates to methods for manufacturing high-pressure discharge lamps used in general illumination, in projectors and automobile headlights in combination with a reflecting mirror, or in like applications.
In recent years, image-projecting apparatuses such as liquid crystal projectors and DMD (Digital Micromirror Device) projectors have been widely used as systems for realizing large-scale video images. In such image-projecting apparatuses, high-pressure discharge lamps with high intensity have been commonly used. FIG. 14 is a schematic view illustrating the structure of a conventional high-pressure discharge lamp 1000. The lamp 1000 illustrated in FIG. 14 is a so-called ultrahigh-pressure mercury lamp, which is disclosed, for example, in Japanese Unexamined Patent Publication No. 2-148561.
The lamp 1000 includes a luminous bulb (arc tube) 101 made of quartz glass, and a pair of sealing portions (seal portions) 102 that extend from both ends of the luminous bulb 101. A luminous material (mercury) 106 is enclosed (in a discharge space) inside the luminous bulb 101, and a pair of tungsten electrodes (W electrodes) 103 made of tungsten are opposed to each other at a predetermined distance. The W electrodes 103 are each welded at one end to a respective molybdenum foil (Mo foil) 104 that is provided in each sealing portion 102, so that the W electrodes 103 are electrically connected with the respective Mo foils 104. The Mo foils 104 are each electrically connected at one end to a respective external lead (Mo rod) 105 made of molybdenum. In addition to the mercury 106, argon (Ar) and a small amount of halogen are also enclosed in the luminous bulb 101.
The operational principle of the lamp 1000 will be briefly described below. When a start voltage is applied across the W electrodes 103 via the external leads 105 and the Mo foils 104, discharge of argon (Ar) occurs. This discharge increases the temperature in the discharge space in the luminous bulb 101, thereby heating and evaporating the mercury 106. The resultant mercury atoms are then exited to emit light in the central portion of the arc between the W electrodes 103. The higher the mercury vapor pressure in the lamp 1000 becomes, the more light is radiated, which means that a lamp with a higher mercury vapor pressure is more suitable as a light source of an image-projecting apparatus. However, in view of the physical strength of the luminous bulb 101 against pressure, the lamp 1000 is used at a mercury vapor pressure of from 15 to 20 MPa (150 to 200 atm).